1. Field of the Invention
The present invention relates to an electronic equipment using a battery as a power source, and more specifically, it relates to an electronic equipment provided with a display unit having a function of displaying the expiration of the lifetime of a battery.
2. Description of the Prior Art
An IC card capable of information processing requires a power source. As the power source, a lithium battery is generally used, but its voltage-fall is a cause of a malfunction of the IC card. Hence, in an IC card with a display unit, the display unit is utilized to display the expiration of the lifetime of the battery (see Unexamined Japanese Utility Model Publication No. 182740/1984).
FIG. 6 shows a structure of a conventional IC card with a display unit, which has a function of displaying the expiration of the lifetime of a battery. Voltage V of a battery 16 energizing an LSI 21 of the IC card is detected by voltage detecting circuits 22 and 23. The first voltage detecting circuit 22 informs beforehand of the expiration of the lifetime of the battery and detects voltage V.sub.1 somewhat lower than a rated voltage V.sub.0 of the battery 16 (see FIG. 7). When the first voltage detecting circuit 22 detects the voltage V.sub.1, the LSI 21 makes a display unit 24 inform beforehand that the lifetime of the battery has almost expired. The second voltage detecting circuit 23 detects voltage V.sub.2 lower than the voltage V.sub.1 to inform of the expiration of the lifetime of the battery 16 (see FIG. 7). As the detection is completed, the LSI 21 makes the display unit 24 display to inform of the expiration of the lifetime of the battery 16. When the voltage V of the battery 16 gets lower than the voltage V.sub.2, the LSI 21 exercises a malfunction.
Such a conventional IC card with a display unit having a function of displaying the expiration of the lifetime of a battery can inform of the expiration of the lifetime of the battery but cannot substantially perform a function of informing beforehand. This is because voltage V of a lithium battery generally employed in IC cards, as shown in FIG. 7, rapidly falls near the expiration of the lifetime, and although the voltage V is detected double-stepwise (V.sub.1, V.sub.2) by the two kinds of voltage detecting circuits 22 and 23, there is little difference between respective detection times t.sub.1 and t.sub.2. Thus, even when the display unit 24 displays to inform beforehand of the expiration of the lifetime of the battery, a trifling delay in battery exchange causes a malfunction in the LSI 21.